High brightness light emitting diodes (HBLEDs) offer enhanced energy efficiency thus making them suitable for specialty lighting applications. An LED device is usually composed of the LED chip fabricated onto a substrate and then encapsulated by a material acting as a lens. The following are the operational requirements of a material to be utilized as an encapsulant of LEDs: optical clarity, high temperature resistant, UV resistant, high refractive index and variable mechanical properties (preferably soft to hard materials).
Encapsulant materials must be optically transparent (greater than 90% transmittance) and should be able to withstand high temperatures, for extended periods of time, without degradation in mechanical and optical performance. The LED device encounters high temperature conditions during the device fabrication (soldering up to 260° C.) and during the actual device operation (around 150° C. for thousands of hours).
Epoxy resins have conventionally been used as a transparent resin for the encapsulation (1, 2). Also, PMMA (polymethylmethacrylate-PMMA), polycarbonate, and optical nylon have been used. However, optical properties of such conventional resins degrade over time. Coloration, or “yellowing”, occurs either by heat induced degradation (heat resistance) or via prolonged irradiation with short wavelength light (ultraviolet-resistance). This results in water entering from the encapsulated portion to disturb performance of LED, and the resin discolors by ultraviolet light emitted from LED to decrease light transmittance of the transparently encapsulated portion. Mechanical degradation of the encapsulant also results in cracking, shrinking or delamination from the substrate. Thus, it is desirable to have an encapsulant system that allows variation of mechanical properties, from soft elastomers to hard plastics. The encapsulant must be hard enough to serve as mechanical support for the LED component, and at the same time must be soft or flexible enough to relieve internal stress during the device fabrication (prevent damage to LED chip or wires) and during temperature cycling (expansion and contraction of materials with different thermal expansion coefficients).
To overcome the above problems, a fluorine-containing cured product in transparent encapsulation of an emission element has been proposed (3). Although, this fluorine-containing cured product has excellent colorless transparency, light resistance and heat resistance as compared with the epoxy resin, but has the problem that adhesion to a material to be encapsulated is poor, and it is liable to peel from the material to be encapsulated. Furthermore, a material of LED chip, specifically a material of an emission layer of LED chip, has high refractive index, specifically refractive index of light in a visible light region, of from 2.5 to 3.0, but the fluorine-containing cured product has low refractive index of light in the same wavelength region. Therefore, the pick-up efficiency of light in the same wavelength region has not always been sufficient in the fluorine-containing cured product.
To solve the above problems, LED encapsulated with a glass prepared with a sol-gel method were proposed (4). This LED makes it possible to reduce hygroscopicity through an encapsulating material and decrease in light transmission due to discoloration of an encapsulating material, and additionally improve heat resistance. However, in the sol-gel glass, fine pores are liable to remain and cracks are easily generated. Therefore, there was the problem that when water enters the fine pores or crack sites, the water disturbs performance of LED. Furthermore, a glass is generally poor in adhesion between a substrate and a wiring metal as compared with a resin. Therefore, there was the problem that water enters from the interface between an encapsulating glass and the substrate or the wiring metal.
It has also been proposed that a low melting glass is heat melted, and LED is transparently encapsulated with the melt (5). However, where a low melting glass is generally heat melted, it is necessary to heat the glass to a temperature of from 400 to 700° C. Therefore, a phosphor used in LED may undergo heat deterioration.
To those problems, a silicone resin (polyorganosiloxane) having excellent heat resistance and ultraviolet resistance is used as a substitute of the epoxy resin. However, silicone resins up to now tend to scar easily, and are not yet sufficient in adhesion, colorless transparency, heat resistance, resistance to moist heat and UV resistance (5, 6, 7, 8, 9).
With the recent development of GaN-based devices which emit short wavelength radiation such as blue light or ultraviolet light, and subsequently white light by combining these light emitting diodes with a fluorescent phosphor, the material requirements for the encapsulant has significantly increased. Materials should be able to withstand exposure to radiation of high intensity without degradation in optical and mechanical properties.
Therefore, there is a need for a simple method of preparing robust LED encapsulants with superior optical clarity, high temperature-resistance, UV-resistance, high refractive index, and with variable elastic properties (preferably soft to hard materials). The present invention allows such properties to be achieved. There is also a need for a simple method of modifying the mechanical properties of LED encapsulants without sacrificing their optical clarity, high temperature-resistance and UV-resistance. The present invention allows such properties to be achieved.